Relaxation Studies Of Poly Research Articles

Relaxation study of Poly(methylmethacrylate) in miscible blends with poly(ethylene oxide) by low-frequency anelastic spectroscopy

Abstract Low-frequency anelastic spectroscopy is used to detect the relaxation of polymer blends. The temperature-dependence energy dissipation (tan Φ ) is measured over several frequencies for poly(methylmethacrylate) (PMMA, M =34,000) and the PEO/PMMA blends with 15, 20 and 25 wt% PEO. Three peaks ( α 1 , α 2 , α 3 ) occur on each curve of blends, corresponding to the glass transition relaxation of PMMA ( α 1 ), the liquid–liquid transition of PMMA ( α 2 ) and the sec-liquid–liquid transition of PMMA ( α 3 ). Both α 1 and α 2 peaks move to low temperature, while the α 3 peak tries to move to high temperature with addition of PEO. A mechanism of competition between the effects of free volume and steric hindrance from PEO chains is advanced to explain the phenomena. It is suggested that the increase in free volume controls the segmental dynamics at low temperature, and the steric hindrance is more and more obvious with the increase in temperature; when the temperature is far above the glass transition of blends, the large-scale coupling movement of PMMA and PEO chains play an important role in the dynamics of the blends.

Relaxation study of poly[(4‐oligodimethyl‐siloxanyl)styrene]s by solid state 29Si NMR

AbstractQuantitative evaluation of spin‐lattice relaxation time (T1) was made at each silicon atom of the side chains by solid‐state 29Si NMR. It was found that the closer the silicon atoms were to main chain, the more the motion was restricted. Such local mobility of side chains was correlated to the gas permeation behavior through the polymer membrane.

Nuclear magnetic relaxation study of poly(ethylene oxide) complexed with lithium trifluoromethanesulfonate

A 19F and 1H NMR spin–lattice relaxation investigation of two PEO–LiCF3SO3 samples, with molar ratios O:Li of 3.5:1 and 9:1, in the temperature range 190–330 K is reported. The 19F NMR measurements from the main focus of the study and these are shown to reflect the properties of the single crystalline complex which is known to be present in PEO–LiCF3SO3 mixtures. A 19F relaxation model is developed in which the motion of the CF3 groups about their threefold axes gives rise to an intramolecular dipolar mechanism but in addition there is a contribution from heteronuclear dipolar interactions between the CF3 groups of the CF3SO3– anions and the CH2 groups of the polymer chain. The simulation of the observed experimental spin–lattice relaxation behaviour is based upon a detailed discussion of the properties of a 19F, 1H model system and involves the analysis of reduced second moment ratios. It is suggested that this type of approach has general applicability in cases where a detailed motional model is not available. Reasonable agreement is found between the simulated and experimental relaxation behaviour.

Dielectric relaxation in poly(ethylene glycols) at 9.83 GHz

AbstractDielectric relaxation studies of poly(ethylene glycols), average molecular weight 200–9000, have been carried out in carbon tetrachloride solutions at microwave frequency, i.e. 9.83 GHz. Average relaxation times τ0 and relaxation times corresponding to segmental motion τ1, and group rotations τ2 have been calculated. The variations of relaxation times with increase in degree of polymerization and temperature have been determined. With the increase in the degree of polymerization the flexibility increases and hence the coiling of the chain increases. At a certain temperature there is a decrease in the relaxation time corresponding to segmental motion, suggestive of some switch‐over mechanism in higher polymers. The relaxation time corresponding to group rotations is independent of temperature and degree of polymerization.

1H two-dimensional nuclear Overhauser effect and relaxation studies of poly(dA).poly(dT)

The structure of poly(dA).poly(dT) in aqueous solution has been studied by using 1H two-dimensional nuclear Overhauser effect (2D NOE) spectroscopy and relaxation rate measurements on the imino and nonexchangeable protons. The assignments of the 1H resonances are determined from the observed cross-relaxation patterns in the 2D NOE experiments. The cross-peak intensities together with the measured relaxation rates show that the purine and pyrimidine strands in poly(dA).poly(dT) are equivalent in aqueous solution. The results are consistent with a right-handed B-form helix where the sugars on both strands are in the C2'-endo/anti configuration. These observations are inconsistent with a proposed heteronomous structure for poly(dA).poly(dT) [Arnott, S., Chandrasekaran, R., Hall, I. H., & Puigjaner, L. C. (1983) Nucleic Acids Res. 11, 4141-4155]. The measured relaxation rates also show that poly(dA).poly(dT) has fast, large-amplitude local internal motions (+/- 20-25 degrees) in solution and that the amplitudes of the base and sugar motions are similar. The motion of the bases in poly(dA).poly(dT) is also similar to that previously reported for poly(dA-dT).poly(dA-dT) and poly(dG-dC).poly(dG-dC) [Assa-Munt, N., Granot, J., Behling, R. W., & Kearns, D. R. (1984) Biochemistry 23, 944-955; Mirau, P. A., Behling, R. W., & Kearns, D. R. (1985) Biochemistry 24, 6200-6211].

13C n.m.r. relaxation study of poly(aspartic acid)

13C n.m.r. relaxation parameters ( T 1, line widths and NOE factors) and chemical shifts were measured in dependence on pH for several samples of poly(α- l-Asp) differing in molecular mass and polydispersity, and for samples of poly(α,β- d,l-Asp) differing in molecular mass. Poly(α- l-Asp) dissolved at pH 9 and acidified to pH 6-4 can appear in two different forms, A and B. In n.m.r. spectra these forms differ mainly by the width of the bands of side chain carbons which is larger in form A. Conditions for a reproducible generation of either of the two forms were not found although the effect of molecular mass (3–6 × 10 4), polydispersity, sample concentration (2.0-0.2 mol l −1), temperature of dissolution (4°C–37°C), concentration of NaCl (0–4 mol l −1) or rate of acidification (1 pH unit s −1 week −1) and rate of mixing were investigated. The dynamics of poly(α,β- d,l-Asp) is almost unaffected by change of pH. The relaxation parameters of poly(α- l-Asp) at pH 9 and 4 differ more in form A than in form B. Analysis of relaxation data for the methine carbon of poly(α- l-Asp) in form A by means of the isotropic model with a log x 2 distribution of correlation times yields a correlation time of 1 ns at pH 4. This indicates that the dynamics of the backbone is dominated by rapid segmental motions even at pH 4. The dependence of chemical shifts on pH indicates that the chemical shift values are determined mainly by the ionization of the carboxyl group in the side chain rather than by a conformational transition. The evaluated relaxation parameters suggest, when compared with those of polypeptides with known conformational behaviour, that the two forms of poly(α- l-Asp) differ in conformation (α-helix, β-structure).

Ultrasonic relaxation studies of poly(methylphenylsiloxane) and its solutions in toluene

Ultrasonic attenuation measurements, covering the frequency range 5–100 MHz and the temperature range 253–298 K, made on two samples of poly(methylphenylsiloxane) of nominal molecular weight 4 × 103(MS 550) and 4.6 × 105(MS 350) and on their solutions in toluene are reported. The acoustic attenuation has been compared with the amplitude predicted for a viscoelastic relaxation process. In the case of the MS 550 polymer a good correspondence between experiment and Navier–Stokes theory was observed. In contrast, the MS 350 polymer exhibited a poor correspondence between experiment and theory. This is found to arise from the presence of a significant amount of unreacted cyclic material which exhibits a conformational relaxation.